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13 October 2013

BOBGRAM issued 13 Oct 2013

WEATHERGRAM
YOTREPS
Issued 13 October 2013
Bob McDavitt's ideas for sailing around the South Pacific.
Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the patterned world of weather maps, so please fine-tune to your place. Dates are in UTC unless otherwise stated.
Where to get the best weather?
If you are planning to sail from Tonga/Fiji/Vanuatu/ New Caledonia to NZ in October/ November, and looking to buddy with someone, then you may be interested in checking out the ICA's "All Points Rally" to Opua, see http://lnk.ie/JO10/e=bobmcd1.bobgram@blogger.com/http://www.islandcruising.co.nz/Some sailors have asked me recently how good are the grib files they are downloading. Well, most of you are looking at data gleaned from the GFS model, and it's good, but not the best. A study done in 2012 shows ECMWF is usually the best. Grib data for EC is not easily available but when you have Internet you can check its latest for the next 10 days for http://lnk.ie/JO11/e=bobmcd1.bobgram@blogger.com/http://bit.ly.ecozSecond best is the UK model, again its grib data is not easily available, but MetService pick the model of the day to feed their regional model , and it is usually EC or UK that they pick, and then it is further processed to give maps at http://lnk.ie/JO12/e=bobmcd1.bobgram@blogger.com/http://bit.ly.7daywxSo check these web sites when planning your trip (and add http://lnk.ie/JO13/e=bobmcd1.bobgram@blogger.com/http://www.swellmap.com to look at the likely swells). And if your grib data agrees with these then all is good, but if not then you need to consider multiple scenarios.

TROPICAL TOPICS
It is a busy time in the Northern Hemisphere tropics with TC PHAILIN being the largest and most intense cyclone to make landfall from Bay of Bengal into India in the last 14 years. TC OCTAVE is off the west coast of North America and re-curving towards Baja California. TC NARI is about to travel from east to west across the Philippines and head for China, and TC WIPHA seems to making a bee line for Japan.

WEATHER ZONES
South Pacific Convergence Zone SPCZ
The SPCZ has been taken to the north of its range by that active squally trough which brought a S/SW change to Fiji/Tonga last Thursday night (as mentioned in my weathergram last week). During this week is should slowly drift south again, reaching Fiji and Samoa by the weekend. ECMWF model has a low forming on the SPCZ near Fiji early next week, and GFS mode, in a run dome today, has a deep low over Vanuatu early next week. Too far away to be sure at this stage, but something to keep aware about.

Sub-tropical Ridge STR
The STR is expected to be near 30S this week, and that means enhanced trade winds on the north end of the traveling highs.
One high is expected to move east along 30S from date to 140W between Monday and Friday.
Another High is expected to travel east along 30 to 32S from east Australia on Tuesday to northern NZ on Thursday , fading east of the dateline on Friday UTC.
A third high is expected to depart for eastern Australia along 32/33S on Friday and reach central NZ on weekend of 19/20 October. This southward shift of the high tracks is a sign that summer is coming and is linked to a similar southward shift occurring in the jetstreams aloft.

NZ/Tasman Sea
This area continues to have a period of unsettled weather with enhanced winds thanks to passing jetstreams. Activity seems to be shifting southwards. One active front is moving across the South Island on Monday and the North Island on Tuesday morning local. A weaker front is expected mainly over the South Island on Friday.

Route Briefings
Tahiti to Tonga:
Up until Thursday UTC this route has easterly wind son the north side of a travelling High , and travel is OK. Then a trough is likely to form on the SPCZ over Cooks by Friday UTC and is expected to deepen as it travels east to Tahiti by Sat/Sun 19/20 . So better to depart Tahiti by 16 Oct UTC to avoid this trough.

Between tropics and NZ
Looking OK to depart any time this week. Winds over Fiji/Tonga may be too strong for comfort by Friday 18 Oct UTC due to the HIGH pressure system south of them by then, so nicer to depart by Thursday UTC.

Between New Caledonia and Australia
Trough crossing the Tasman on Monday and Tuesday, then OK to go. Winds over New Caledonia May become strong this weekend or early next week so try and depart before then. Next significant trough expected for Brisbane should reach there around Tue 22 October.